Methods for the preparation of nanoparticle metal and metal oxide colloidal dispersions (or hydrosols) are well known in the art. They include, but are not limited to, (1) the synthesis of colloidal dispersions of various transition metals in aqueous media, stabilized by added polymers as protective colloids (Bawendi et al., Annu. Rev. Phys. Chem. (1990), 41, p. 477); the synthesis of ultrasmall metal oxide particles by the combination of water and metal chloride, hydroxides, or acetates, in aqueous media (Henglein, Top. Curr. Chem. (1988), 143, p.113); the synthesis of Ag nanoparticles by the reduction of Ag.sup.+ in aqueous media (Wang et al., J. Phys. Chem. (1991), 95, p. 525); the formation of colloidal silver and gold in aqueous media by ultrasonic radiation (Hoffman et al., J. Phys. Chem. (1992), 96, p.5546; the formation of colloidal gold in aqueous media by the reduction of a gold salt (Colvin et al., Nature (1994), 370, p.354); the formation of colloidal platinum and palladium in aqueous media by synthetic routes analogous to those for preparing gold colloids (Heath et al., Appl. Phys. Lett. (1994), 64, p.3569). Layers containing colloidal silver are used for long time as filter layers in photographic colour materials.
In WO 97/24224 a method involving reduction is disclosed for the preparation of organically functionalized nanoparticles of metals and metal alloys. For instance, organically functionalized Co--Au alloys can be prepared by this method. In ES 2083309 the preparation by reduction in a medium of micromulsions of ultrafine particles of alloys and magnetic oxides is disclosed. Both teachings cited above show the disadvantage of being cumbersome and of requiring the use of organic solvents.
As disclosed in J. Am. Chem. Soc. (1991), Vol. 113, nanoscale metal particles and mixtures thereof are prepared by reduction in an aprotic organic solvent.
According to U.S. Pat. No. 5,620,584 metals and metal alloys can be prepared by electrochemical reduction.
In pending European patent application, appl. No. 98201117 a process is disclosed for the preparation of a particular heat mode recording element involving the coating from an aqueous medium of a layer containing highly dispersed metal particles.
Heat mode recording materials are capable of producing an image by the transformation of image-wise applied laser light into a corresponding heat pattern. Different types exist according to their composition.
In a particular type of heat mode elements, e.g. as disclosed in EP 0 674 217, density is generated by image-wise chemical reduction of organic metal salts, preferably silver salts such as silver behenate, without the presence of catalytic amounts of exposed silver halide such it is the case in the dry silver system.
Another important category of heat mode recording materials is based on change of adhesion, e.g. as disclosed in U.S. Pat. Nos. 4,123,309, 4,123,578, 4,157,412, 4,547,456 and PCT publ. Nos. WO 88/04237, WO 93/03928, and WO 95/00342.
In still another particular type of thermal recording or heat mode recording materials information is recorded by creating differences in reflection and/or in transmission on the recording layer. The recording layer has high optical density and absorbs radiation beams which impinge thereon. The conversion of radiation into heat brings about a local temperature rise, causing a thermal change such as evaporation or ablation to take place in the recording layer. As a result, the irradiated parts of the recording layer are totally or partially removed, and a difference in optical density is formed between the irradiated parts and the unirradiated parts (cf. U.S. Pat. Nos. 4,216,501, 4,233,626, 4,188,214 and 4,291,119 and British Pat. No. 2,026,346)
The recording layer of such heat mode recording materials is usually made of metals, dyes, or polymers. Recording materials like this are described in "Electron, Ion and Laser Beam Technology", by M. L. Levene et al.; The Proceedings of the Eleventh Symposium (1969); "Electronics" (Mar. 18, 1968), P. 50; "The Bell System Technical Journal", by D. Maydan, Vol. 50 (1971), P. 1761; and "Science", by C. O. Carlson, Vol. 154 (1966), P. 1550. Recording on such thermal recording materials is usually accomplished by converting the information to be recorded into electrical time series signals and scanning the recording material with a laser beam which is modulated in accordance with the signals. This method is advantageous in that recording images can be obtained on real time (i.e. instantaneously). Recording materials of this type are called "direct read after write" (DRAW) materials. DRAW recording materials can be used as a medium for recording an imagewise modulated laser beam to produce a human readable or machine readable record. Human readable records are e.g. micro images that can be read on enlargement and projection. An example of a machine readable DRAW recording material is the optical disc. To date for the production of optical discs tellurium and its alloys have been used most widely to form highly reflective thin metal films wherein heating with laser beam locally reduces reflectivity by pit formation (ref. e.g. the periodical `Physik in unserer Zeit`, 15. Jahrg. 1984/Nr. 5, 129-130 the article "Optische Datenspeicher" by Jochen Fricke). Tellurium is toxic and has poor archival properties because of its sensitivity to oxygen and humidity. Other metals suited for use in DRAW heat-mode recording are given in U.S. Pat. No. 4,499,178 and U.S. Pat. No. 4,388,400. To avoid the toxicity problem other relatively low melting metals such as bismuth have been introduced in the production of a heat-mode recording layer. By exposing such a recording element very shortly by pulses of a high-power laser the writing spot ablates or melts a small amount of the bismuth layer. On melting the layer contracts on the heated spot by surface tension thus forming small cavitations or holes. As a result light can pass through these cavitations and the density is lowered to a certain Dmin value.
According to EP 0 384 041 a process is provided for the production of a heat mode recording material having "direct read after write" (DRAW) possibilities wherein a web support is provided with a heat mode recording thin metal layer, preferably a bismuth layer, characterized in that in the same vacuum environment a protective organic resin layer in web form is laminated to said supported recording layer by means of an adhesive layer.
A commercially available material manufactured according to the principles of cited EP 0 384 041 is MASTERTOOL MT8, registered trade name, marketed by Agfa-Gevaert N.V.
A drawback of the method of preparation of a thin bismuth recording layer by vacuum deposition is the fact that this is a complicated, cumbersome and expensive process. Therefore, in pending European patent application appl. No. 98201117, cited above, an alternative process for applying a thin metal layer is described comprising the following steps:
(1) preparing an aqueous medium containing ions of a metal, PA1 (2) reducing said metal ions by a reducing agent thus forming metal particles, PA1 (3) coating said aqueous medium containing said metal particles on said transparent support. PA1 (1)preparing an aqueous medium containing at least two different types of metal ions, belonging to a group chosen from the groups I B, II B, III A, IV A, and V A from the periodic system of elements, PA1 (2)chemically reducing said metal ions by a reducing agent.
In a preferred embodiment the metal layer is again a bismuth layer. However, such bismuth layers coated from an aqueous medium suffer in their turn from another drawback. Compared to bismuth layers prepared by vacuum deposition their sensitivity to laser light is lower. This is due to the presence of a higher degree of oxidized bismuth, and to the presence of ballast compounds in the layer such as a binder and additives improving stability, which to a certain degree hamper the formation of microspheres by the action of laser radiation.